With the gradual decrease of optical transmission costs, it is an evitable development trend that the access network will be based on fibers. The access network segment that represents the “last mile” is required to be characterized by ultra-low costs, simple structure, and easy implementation, which brings great challenges to technical implementation. A Passive Optical Network (PON) employs passive components, and is the most potential technology for implementing broadband optical access network.
PONS are generally composed of an OLT located at the central office and a series of ONUs located at the customer premises. An Optical Distribution Network (ODN) composed of fibers and passive optical splitter or coupler exists between the central office and the customer premises. This mode makes multiple users share the relatively expensive fiber link from the central office to the customer premises, which greatly reduces the cost of implementing Fiber To The Building (FTTB) and Fiber To The Home (FTTH).
Therefore, it is essential to monitor and maintain the fiber link between the OLT at the central office and the ONU at the customer premises. In practice, a mode of monitoring and maintaining the fiber link is: The ONU at the customer premises sends a burst optical signal to the OLT at the central office, and the OLT at the central office receives the burst optical signal and measures the power of the burst optical signal, and then analyzes and monitors the performance of the fiber link between the OLT and the ONU according to the power value of the burst optical signal.
When the OLT measures the power of the burst optical signal sent by the ONU, the OLT needs to first sample current mirrors for the optical current corresponding to the burst optical signal sent by the ONU to obtain the mirrored current of the optical current, convert the mirrored current into voltage, hold the samples of voltage, use an Analog-to-Digital Converter (ADC) to convert the held voltage to digital signals, and calculate optical power. Normally, it takes about 100 us to finish the process from sampling the optical current to calculating out the optical power value. In the time of 100 us, the ONU must continuously send optical signals in order to eventually obtain the high-precision optical power value. To ensure the ONU to keep sending the burst optical signals for a certain period such as 100 us, the OLT needs to allocate a wide bandwidth to the ONU that is to be measured. The wide bandwidth, for example, at least 100 us, is relatively wide as against the timeslot for the ONU to send data normally. In the process of allocating wide bandwidth to the ONU that is to be measured, it is required that a Dynamic Bandwidth Assignment (DBA) module adjusts bandwidth for all ONUs in order to allocate a wide bandwidth to the ONU that is to be measured.
In the research process, the inventor of the present invention finds that when the DBA module of the OLT assigns bandwidth to the ONU, a certain update period exists. That is, the DBA module does not update or adjust the bandwidth occupied by each ONU in each frame until every m frames have elapsed, where m is an integer greater than 1 and is generally an integer multiple of 2. As shown in FIG. 1, the update period of the DBA is 8 frames. In non-measurement periods, the bandwidth occupied by ONU1 in one frame (125 us) is only M us, for example, M=25. ONU1 transmits data in the 25 us time. For the purpose of measuring the Received Signal Strength Indicator (RSSI) for ONU1, a bandwidth of a continuous time bucket needs to be allocated to ONU1 and the continuous time bucket is greater than the 25 us in each frame (125 us), for example, bandwidth of at least 100 us. In order for the DBA module to allocate bandwidth of at least 100 us to ONU1, the bandwidth occupied by other ONUs in the same OLT will decrease accordingly. The bandwidth occupied by each ONU keeps unchanged in an update period of a DBA, namely, the DBA allocates bandwidth of at least 100 us to ONU1 in each frame in an update period of the DBA. Therefore, only the remaining bandwidth in each frame is available to other ONUs in the update period of the DBA. As shown in FIG. 1, during the period of measuring the RSSI, within an update period (8 frames) of the DBA, ONU1 occupies at least 100 us in each frame. In practice, it takes only 100 us to measure the optical power. That is, the bandwidth allocated in the first frame within an update period of the DBA can meet the requirement of measurement; the other 7 frames do not need to perform measurement of optical power, and are used for transmitting the data normally; but ONU1 requires bandwidth of only 25 us during the time of transmitting data normally. Therefore, within an update period of the DBA, ONU1 wastes bandwidth of at least 75 us*7=525 us; and the percentage of the wasted bandwidth is 525 us/(125 us*8)=52.5%, but the bandwidth required by other ONUs is not satisfied. In the prior art, the method that the DBA module is used for allocating wide bandwidth to the ONU that is to be measured for the purpose of measuring burst optical power causes tremendous waste of bandwidth and affects the work of other ONUs.